In machines for industrial use typified by machine tools, drive control is performed so that tools or tables follow command values. In particular, control for driving a machine so that the tool position relative to a workpiece that is an object to be machined, to accurately follow a commanded trajectory, which is a path of a tool specified by a command, is called trajectory control or contour motion control. The control is performed precisely using a numerical control device and a servo control device attached thereto. A machine to be controlled has a plurality of movement axes, and is driven by the servo control device in directions along the movement axes.
The numerical control device performs position control in the movement axis directions of the machine by using the servo control device. Feedback control is basically performed so that a feedback position detected by a motor end encoder or a linear scale for measuring the position of a tool end follows a command position. However, because error factors such as backlash and lost motion are present in a mechanical system between the feedback position and the tool end position, the tool end position has error with respect to the feedback position due to the influence of the error factors. Backlash refers to clearances of ball screws or the like. Lost motion refers to elastic deformation of ball screws caused by friction on guiding surfaces. The presence of the error of the tool end position with respect to the feedback position means that error occurs at the tool end position when feedback control alone is performed.
In particular, at reversal of the direction along a movement axis of a feed shaft, the movement trajectory of the tool posit iron is significantly affected by backlash or lost motion. A typical example is stepped trajectory error caused at a boundary of quadrants of an arc trajectory when an instruction of an arc trajectory is made. This is because the trajectory is deviated in the negative direction during movement in the positive direct iron and the trajectory is deviated in the positive direction during movement in the negative direction by the influence of the backlash or the lost motion of the mechanical system. Occurrence of such error in following a trajectory causes a line or a scratch on a machined surface, which is not desired.
To reduce the influence of such error factors as backlash and lost motion present in a mechanical system, the numerical control device therefore has a function of predicting error of a trajectory caused by the error factors of the mechanical system and correcting the predicted error in addition to the feedback control. In particular, so-called backlash compensation in which a correction value is added to a command position is performed when the direction of the command position is reversed, that is, when the posve/negative sign of the speed is inverted, in order to reduce the influence of backlash and lost motion (refer to Patent Literature 1, for example).